In Fig.5.5, the compensator G may be characterized by either as a diagonal or as a non-diagonal matrix. The use of a fully populated (non-diagonal) matrix allows the control system designer much more design flexibility to achieve the maximum number of the desired performance specification 9see the last paragraph of Sec. 7-9). In other words, the use of non-diagonal elements of G may ease the diagonal compensator design problem. Unless otherwise stated a diagonal G is used in Chapters 5 - 9. Chapter 10 discusses the use of a non-diagonal compensator for a MIMO QFT design.